1. Related Applications
The application relates to the invention described in our co-pending application Ser. No. 314,975 filed Oct. 26, 1981, and entitled "A Magnetoresistant Transducer for Reading a Record Carrier Having a High Data Density". The subject matter of said application is hereby incorporated by reference.
2. Field of the Invention
The present invention relates to magnetoresistant transducers and more particularly to magnetoresistant transducers for reading data on multi-track magnetic carriers, such as rigid or flexible magnetic discs and magnetic tapes in which the data density is very high.
3. Description of the Prior Art
It is known that magnetic discs carry data on circular concentric recording tracks which have a radial width no greater than a few hundredths of a millimeter and which commonly cover the greater proportion of both their surfaces, while magnetic tapes carry data on tracks parallel to the tape length. As a rule, a data recorded on a track of a magnetic disc or tape appears in the form of a succession of small magnetic areas referred to as "elementary areas" which are distributed throughout the track length and have magnetic inductions of identical modulus and opposed direction.
The term longitudinal density (or linear density) is used to define the number of data per unit of length measured along the circumference of a track in the case of a magnetic disc, or along the tape length in the case of a magnetic tape.
The means which make it possible either to record data on disc or tapes, or to read data, or finally to implement one or the other of these functions, are referred to as magnetic transducer devices. As a rule, one or more magnetic transducer devices is or are associated with a given carrier, the carrier traveling past and in front of the transducer device or devices.
Increasingly, frequent use is being made of transducer devices which comprise one or more magnetoresistances to read data on discs or tapes. The transducers are denoted by the name "magnetoresistant transducers." It will be recalled that magnetoresistances are electrical resistances having the form of thin layers or films of very small thickness of which the length greatly exceeds the width. The term "thin layer" as used herein denotes a layer having a thickness in the range of several hundred Angstrom to several microns.
These magnetoresistances are frequently deposited on a substrate of electrically insulating material. The value of their resistance varies when they are exposed to a magnetic field.
Consider, for example, a measuring magnetoresistance R connected to the terminals of a generator which supplies a current I flowing in the direction of its length, and assume that the magnetoresistance relates to a magnetoresistant transducer associated with a magnetic record carrier and that it is positioned at a very small or non-existent distance from the carrier. When each of the elementary magnetic areas of the carrier passes before the transducer, the magnetic leakage field H.sub.f generated by these areas close to the carrier surface causes a variation .DELTA.R of the resistance R and therefore a variation V=I.times..DELTA.R at its terminals, which yields .DELTA.V/V=.DELTA.R/R, .DELTA.R/R being referred to as the "magnetoresistance coefficient". This coefficient is commonly of the order of 2% and is very frequently negative.
The electrical signal collected at the terminals of a magnetoresistance has an amplitude independent of the speed of the record carrier.
It will be recalled that the expression "initial magnetic permeability of a magnetic material" is defined as the ratio (B/H) between the induction and the magnetic field when B and H are close to zero on the initial magnetization curve. The initial magnetization curve is the curve defining the variation of B as a function of the magnetic field H when the magnetic material is exposed to a magnetic magnetization field starting from an initial magnetic state of the material defined by B and H being close to zero. In other words, the initial magnetic permeability of the material is equal to the slope of the initial magnetization curve close to the point B=O and H=O.
It will also be recalled on the other hand that a magnetically isotropic material situated in a plane (which means that its thickness is much smaller than its length and also than its width) has two preferential directions of magnetization which are commonly at right angles to each other, in said plane. One of these is referred to as the "direction of easy magnetization" whereas the other is referred to as the "direction of difficult magnetization". The initial permeability of the material in the direction of difficult magnetization is much greater than the initial permeability of the material in the direction of easy magnetization.
The expression anisotropy field H.sub.k is used to denote the value of the magnetic field H applied to the material in its direction of difficult magnetization, for which the said material is saturated in this direction.
As a rule, the magnetoresistances utilized are formed by a magnetically anisotropic material, for example by an iron-nickel alloy (18% of iron and 82% of nickel). Their axis of easy magnetization is parallel to the direction of the current I and to their length, whereas their axis of difficult magnetization is at right angles to the same. The position of the magnetoresistance(s) of a magnetoresistant transducer compared to the record carrier allocated to it (them), is such that the leakage field of the elementary areas is parallel to its (their) axis of difficult magnetization, which is itself at right angles to the surface of the carrier. If the magnetoresistances are not exposed to any magnetic field, their magnetization (that is to say the magnetic induction within the same) is directed along the direction of the axis of easy magnetization.
It can be demonstrated that it is possible to increase the sensitivity of a magnetoresistance formed by an anisotropic magnetic material, that is to say the voltage of its output signal, as a function of the magnetic field to which it is exposed, by exposing the same to a magnetic polarizing field H.sub.pol parallel to its axis of difficult magnetization, as specified in U.S. Pat. No. 3,945,038 under the title: "Improved magnetoresistances and electromagnetic transducer incorporating the same".
The value of the polarising field H.sub.pol is selected in such a manner that it causes the magnetization in the magnetoresistances to turn through an angle .theta. preferably close to 45.degree. (in this case, the magnetization subtends an angle of 45.degree. with the direction of easy magnetization).
In this case, it is demonstrated that the sensitivity of the magnetoresistance is at a maximum, that is to say that a maximum variation of its resistance and consequently of its output voltage corresponds to a given variation .DELTA.H of the magnetic field to which it is exposed (other than the field H.sub.pol).
In current practice, magnetoresistant transducers comprise two parallel magnetoresistant elements (that is to say, their lengths are parallel) separated by a distance of the order of a tenth of a micron. This distance is substantially smaller in any event than the length of the elementary magnetic areas present on each recording track of the magnetic carrier, so that these two magnetoresistances are exposed to a magnetic leakage field generated by the area before which they are positioned, which has the same value.
The two magnetoresistant elements are polarized in such a manner that their magnetizations are turned through an angle of 45.degree. and are at approximately 90.degree. to each other, as set forth in U.S. Pat. No. 3,942,889. The output signal .DELTA.v.sub.1 of the first magnetoresistant element is fed to a first input terminal of a differential amplifier, whereas the output signal .DELTA.v.sub.2 supplied by the second magnetoresistant element, is fed to the second input terminal of the same differential amplifier. Since .DELTA.v.sub.1 is substantially equal to -.DELTA.v.sub.2, a signal which if proportional to 2.times..vertline..DELTA.v.sub.1 .vertline. is collected at the output terminals of the two differential amplifiers. The utilization of a differential amplifier renders it possible to effect a substantial reduction of the noise signal as compared to the signal proportional to 2.times..DELTA.v.
The noise signal may be attributed in particular to thermal disturbance in the magnetoresistances, and equally to all the magnetic fields other than the magnetic leakage field generated by the area opposite which the two magnetoresistances are placed.
It is evident that the two magnetoresistant elements are exposed not only to the magnetic leakage field of the area with which they are in alignment, but equally to the resultant of the magnetic leakage fields generated by the magnetic areas situated at either side of the area opposite which these two magnetoresistances are located. If this resultant has a comparatively low value compared to the value of the magnetic leakage field of this area, when the linear data densities are comparatively low, this does not apply when these linear densities are substantial. In these circumstances, the said resultant may be comparatively substantial compared to this magnetic leakage field. It is then necessary to position magnetic screening devices, commonly formed by a set of thin blades of magnetic material which are interconnected and separated by thin non-magnetic layers, at either side of the two magnetoresistant elements. The plane of each of these blades is at right angles to the record carrier and to the direction of travel of the tracks.
The blades forming the magnetic screening devices preferably consist of anisotropic magnetic material. Their axis of difficult magnetization are perpendicular to the magnetic carrier, so that the totality of the lines of the magnetic field generated by the areas enflanking the magnetic area opposite which the magnetoresistances are located, is intercepted by the blades and not by the two magnetoresistant elements.
Each magnetoresistant element of a magnetoresistant transducer of this kind is preferably polarized or biased by the magnetic field generated by the passage of the current through the other magnetoresistant element. Thus, if H.sub.1 is the magnetic field generated by passage of the current I through the first magnetoresistant element, the second magnetoresistant element is polarized by this field H.sub.1, and conversely if the field H.sub.2 is generated by the same current I in the second magnetoresistant element, the first magnetoresistant element is polarized by the field H.sub.2. It is obvious that, as a rule, H.sub.1 is substantially equal to H.sub.2 in absolute value and of opposite sign. It is then sufficient to adjust the intensity of the current I flowing through the two magnetoresistant elements in such a manner that the two elements are each polarized to an angular value of the order of 45.degree. and that the magnetizations in each of these magnetoresistances are then situated at 90.degree. with respect to each other.
In the case in which the magnetoresistant transducers having two magnetoresistant elements comprise magnetic screening devices situated at either side of the latter, the following actions occur:
the magnetic screening devices situated beside the first magnetoresistant element are exposed to the magnetic field H.sub.1 generated by the passage of the current I through this element. This field H.sub.1 in its turn generates, within the magnetic screening devices, a volumic and surfacial distribution of charges respectively at the inside and on the surface of the section of these screening devices exposed to the field H.sub.1. The magnetic charges are greater in number the larger the volume of the magnetic screening devices exposed to the field H.sub.1 and the greater the intensity of this field. A more detailed explanation of this action may be found in the book by W. F. BROWN under the title "Principes de Ferromagnetisme" in chapters II and III, published by Editions Dunod in 1970, and equally in the book by Duranc, in chapter VI, sub-section I, para. 3, page 302, and chapter VIII, published by Masson in 1968.
It is evident that identical actions occur in the magnetic screening devices situated beside the second magnetoresistant element and exposed to the field H.sub.2 generated by passage of the current I through this element. The magnetic charges generated within the magnetic screening devices (assuming these to be the first magnetic screening devices situated beside the first magnetoresistant element, the actions produced by the other magnetic screening devices obviously being identical) generate for their part a magnetic field referred to as a "magnetic return field" which tends to oppose the magnetic field H.sub.1 which had generated the said charges. H.sub.r denotes return field. The absolute value of H.sub.r is substantially equal to a third of the field H.sub.1, equally considered in absolute value. It is then apparent that, in these circumstances, the polarizing field of the second magnetoresistant element is no longer H.sub.1 but H.sub.1 -H.sub.r. It is equally demonstrable that the magnetic polarizing field of the first magnetoresistant element is no longer H.sub.2 but H.sub.2 -H.sub.r. The magnetoresistances are no longer polarized at 45.degree. , but at an angle of lesser value, which has the result of reducing their sensitivity (that is to say the ratio .DELTA.R/.DELTA.H) compared to what it had been when they were polarized at 45.degree.. Furthermore, the signals delivered are no longer linear. It is obviously possible to eliminate this disadvantage by raising the current intensity in the two magnetoresistances to increase the intensities of the fields H.sub.1 and H.sub.2 to regain a polarization angle of the magnetoresistances of the order of 45.degree., but this implies excessive heating of the magnetoresistances on the one hand, and on the other hand requires an increase of the power needed to polarize these.